Hi ARD, welcome to the forum.
There is a lot of sound quality to be gained from a decent digital transport - this is the whole ethos of Innuos with quality power supplies, boards, bespoke firmware and so on.
With your ZENith, you can use the native Sense app from which you may find you get the best sound quality results. You do have the option to switch it into a Roon Core if you wish to continue with that, and you will likely hear a difference in sound.
At the moment there is no support for Audirvana, but we are looking in to this as a possibility for future software development, so it could well appear one day.
In the Sense app there will be a facility to import from a NAS which i suspect will be the easiest way to transfer your 1200 AIFF albums. Sense app is quite like Roon in that it is a unifying library that combines your local files, Tidal, and Qobuz, so indeed should be a one-stop shop.
If using the Sense app, you can also go into its Audio Settings and try switching the USB Latency to Low mode for a potential boost in SQ.
Anyway, let us know how you get on and happy to answer further questions.